The present invention generally relates to a wireless communication system and, more particularly, to a user terminal power control system with a predictive closed-loop and adaptive correlation open-loop approach.
There are presently many different types of radiotelephones or wireless communication systems, including different terrestrial based wireless communication systems and satellite communication systems.
The CDMA2000, an exemplary cellular system, is a communication protocol and specifies the technical requirements that form a compatibility standard for CDMA systems. One of the technical requirements specified in CDMA2000 is allocating one bit per data package (or, equivalently, frame) to specify the status of signal power strength. FIG. 1 is schematic diagram of a typical terrestrial wireless communication system 100, where user terminals 104, such as cellular phones, Personal Digital Assistants (PDAs), etc., communicate with each other via a gateway station 102.
In the communication system 100, each user terminal 104 exchanges a data package (or, equivalently, frame) with the base station 102 at each time interval that is about 20 milliseconds in CDMA2000 system. To conform to the CDMA2000 requirements, each data package contains one bit dedicated to indicate the received signal quality (or, equivalently, transmission power level). The data contained in the one bit may be used by the receiver to adjust the transmission power level of the following data package. Thus, one cycle of data exchange makes a closed feedback loop to adjust the transmission power levels of the communicating user terminals 104. The adjustment of signal level is required to minimize the interference between the user terminals 104 and, consequently enhance the quality of communications therebetween. A weak signal level fails to provide clear communication between each user terminal 104 and the base station 102, while an excessively strong signal from one user terminal may produce an undesirable interference to other user terminals. As the power control cannot be perfect, for each communication system design, some additional power allowance is placed to allow the power control error. The additional power allowance is known as power control cushion. It is clear that for power use efficiency and for system reaching a high performance and high capacity level, the power control cushion should be as small as possible.
In the terrestrial wireless communication system 100, the time delay of the closed feedback loop can be just a matter of tens of milliseconds and, as a consequence, the closed feedback loop power control can promptly adjust its power level to adapt the environmental change. In contrast, the time delay of the closed-loop methods for existing satellite communication systems can be significant. For example, in a typical GEO satellite communication system, the feedback closed-loop delay is about ¼ second for a single hopping method and about ½ second for a double hopping method. With such a long feedback time delay, a well-designed conventional closed-loop method by itself may fail to provide a satisfactory power control and at least a 2-dB power control cushion is required.
In the design of a satellite system, closing the link budget while providing the largest possible transmission capacity has been a challenging problem to engineers. In addition, the transmission power limitation may compound the difficulty of the problem. Analysis shows that in a GEO Mobile system, with other resources the same, a reduction of the power control cushion by 1-dB can double the system capacity.
Therefore, there is a need to complement the long loop delay effect in the power control system so that the power control cushion can be reduced and, consequently, the system capacity can be increased and the quality of service can be enhanced.